After a voice broadcast call was answered by a machine/voicemail, no voicemail was left.

Possible Causes:

Seen frequently when testing broadcasts, if the caller-ID used in the broadcast is the same as the number dialed, likely the call did not reach the voicemail box because it appears to the carrier that your phone is calling itself. Many carriers have configured their network such that when a phone calls itself (i.e. the calling-party is the same as the called-party), the call is redirected to the voicemail main menu where the caller is prompted to enter their password in order to check their messages. Since this is a different menu than the voicemailbox where a message would be deposited, no message is left in the voicemailbox.

Some carriers have configured their voicemail systems to discard any voicemail messages which contain too much silence at the beginning of the message. Some possible reasons why too much silence might exist at the beginning of a voice message is discussed in another article, but the simplest solution is to remove any silence which exists at the beginning of the voice message which is sent on the broadcast.